HWC2: Clarify null client target handle

Documentation-only change that clarifies that in the case where the
client isn't compositing any of the layers, the buffer handle
provided to setClientTarget may be null, and that this should not be
treated as an error by the device.

Bug: 28569722
Change-Id: Ia5825eb0a80c7f838894642654db7d7be14a3b6b
1 file changed